There are several means known in the prior art for the hard coating of metal substrates with thin, decorative and wear-resistant coatings. Such coatings include titanium nitride, titanium carbide, titanium carbonitride, zirconium nitride, zirconium carbide, and zirconium carbonitride. Such hard coating techniques include evaporative vacuum deposition, electroplating, magnetron sputtering, ion plating, and thermal or plasma spray techniques.
Substrates to be hard coated, particularly for use as tools or instruments, are often made of steel. The coating, for example, of a high speed steel drill bit with a thin coating of titanium nitride can be accomplished so that the drill bit has a decorative golden finish that provides excellent wear characteristics.
The steel substrate typically used for tools or instruments that will be hard coated generally has a "rough" surface. Although the surface of the substrate will appear smooth upon visual inspection, the surface is pitted and rough on a microscopic level. In addition to the surface "roughness", the substrate surface contains various small deposits of oxides from the manufacturing process. These surface irregularities are not removed in the manufacturing process. Although chemical cleaning by reverse plating or acid etch is possible to remove these deposits, such a process would be costly and environmentally troublesome. The first stage of any hard coating process is the "cleaning" of the substrate surface in order to allow for the application of a conformal coating.
Effective and efficient applications of hard coated metal surfaces dictate that extremely thin layers of the metal compound be applied to the substrate surface. It is possible to coat such substrates with surfaces of less than two or three microns in thickness. Notwithstanding the pitted nature of the steel substrate, the existence of various surface deposits of oxides, the thinness of the applied coating and the characteristics of the metal compounds being applied, it is generally possible to place a hard coating on the substrate that is essentially free of fissures or pores.
These various factors do combine, however, to create a variety of coating defects. One example of a coating defect is the existence of a nodule. Nodules are cone-shaped particle formations in the surface of the coating. Nodule boundaries have increased permeability properties. Another example of a common surface defect is referred to as "bridging". Small amounts of free iron on the surface of the substrate, along with some adsorbed oxygen, resist coating and create bridges or areas of relatively thin coating. These areas of thin coating also have increased permeability properties.
The existence of these and other types of defects in the hard coated metal compound is not detectable by visual inspection, and does not in any way affect the wear-resistant characteristics of the coating. The existence of coating defects does lead to surface phenomena that eventually tend to the formation of corrosion on the surface of the substrate.
Corrosion can occur in two ways. The simple oxidation of iron leads to rust formation very rapidly. Corrosion can also result from the formation of "galvanic cells" between the coating and the substrate. Galvanic cells are created at the point of conductive contact between two dissimilar metals. Galvanic cells are created between the coating and substrate at defect sites due, in part, to the migration of iron through the coating and the presence of air and moisture migrating through the coating to these sites. No amount of surface preparation or cleaning prior to the coating process has been found that will prevent the formation of galvanic cells--and therefore corrosion--at the surface/coating interface. Such corrosion leads rapidly to large areas of the hard coated article becoming blotched and darkened. Although the wear-resistant characteristics of the hard coated metal compound article are not affected, the decorative surface is marred.
Parylene is a uniform conformal polymeric coating material that is used primarily in the electronics industry. One of its most important properties is its ability to form an extremely thin layer onto the surface of substrates and to conform uniformly--over sharp edges, points, flat surfaces, crevices or exposed internal surfaces--to the surface of the coated substrate. Parylene is also valuable for its ability to resist many chemicals and relatively high temperatures.
Parylene is a generic term applied to the family of unsubstituted and substituted poly-p-xylylenes. The specific parylene polymer formed can be either a homopolymer or a copolymer, depending on the particular building blocks used to create the polymer. The parylene polymer is generally produced from one or a mixture of para-xylylene dimer compounds. The unsubstituted homopolymer poly-p-xylylene has the structure ##STR1## which is referred to by its trademark name of PARYLENE N. Other parylenes which can be produced from commercially available dimers are as follows: ##STR2## Of course, an almost unlimited number of substituted co- and homopolymers have been suggested in the prior art.
A description of parylenes, the processes for making them, and the apparatus in which parylene vacuum deposition can be effected may be found in U.S. Pat. Nos. 3,246,627 and 3,301,707 of Loeb, et al. and U.S. Pat. No. 3,600,216 of Stewart, all of which are incorporated by reference herein. These patents do not utilize the term "parylene", but instead refer to poly-p-xylylenes.
The typical process for coating any given substrate with a thin layer of parylene involves the vacuum vapor deposition of the biradical monomer units that are produced by pyrolysis of dimeric para-xylylene units as follows: ##STR3## The biradical monomers formed are then introduced into a deposition zone where they rapidly polymerize on the surface of an ambient temperature substrate.
By appropriate control of reaction conditions, the desired thickness of parylene coating may be obtained. The polymeric coating formed is almost exclusively linear, i.e., without any cross-linking between the linear polymeric chains. In some instances, the substrate surface is first treated with a silicone-based compound in order to enhance the binding of the parylene to the substrate. See U.S. Pat. Nos. 3,600,216 of Stewart and U.S. Pat. No. 4,225,647 of Parent.
The ability of parylene to form a conformal coating is so efficient that the application of a layer of parylene creates a barrier from the environment to the substrate. The ability to form an environmental barrier is a well known property of parylene that finds many uses, particularly in the electronics industry.
Unfortunately, despite the varied desirable qualities of a parylene coating, the thin parylene coating is quite soft and is easily worn off of any substrate that is subject to wear or moderate physical handling. Because of this, all commercial applications for parylene, to date, are limited to the protection of substrates that are not subject to any significant physical wear.
Therefore, even though hard coated metal substrates and parylene coated substrates both have many significant applications, neither can be used satisfactorily when a hard, thin, wear-resistant, decorative and corrosion free surface is required on metal substrates that are subject to even moderate amounts of physical wear. The soft surface provided by a parylene coating is a well known liability of such a coating. For this reason and others it is not obvious to apply a soft parylene coating over a hard wear-resistant coating.